Referring to FIG. 1, a conventional vehicle brake system includes an air compressor 11, an air reservoir 12 receiving pressurized air (i.e., pressurized fluid) from the air compressor 11, a conduct line 13 connected to the air reservoir 12 and a relay valve 14, a plurality of actuators 15 (only one is shown in the figure) connected to the relay valve 14, a first brake line 16 connected to the air reservoir 12 and a brake pedal 17, and a second brake line 18 connected to the brake pedal 17 and the relay valve 14.
In a normal driving situation, the pressurized air is delivered from the air reservoir 12 to the relay valve 14 and the brake pedal 17 through the conduct line 13 and the first brake line 16, respectively. At this time, the pressurized air is not delivered between the brake pedal 17 and the relay valve 14, thus the actuators 15 are not actuated and a brake action is not generated.
When a vehicle operator steps on the brake pedal 17, the pressurized air is delivered to the relay valve 14 through the second brake line 18 to thereby permit the pressurized air in the conduct line 13 to flow to the actuators 15 for generating a brake action.
However, for generating a brake action in rear wheels of a long vehicle, such as a bus, the second brake line 18 has to be relatively long for proper operation of the relay valve 14, which leads to a long response time and results in a time delay of the brake action. In addition, the amount of the pressurized air flowing from the conduct line 13 to the actuators 15 depends on an extent of coverage of an opening (not shown) of the relay valve 14, which is determined by the amount of the pressurized air flowing into the relay valve 14 from the second brake line 18. The control of the pressurized air flow in the relay valve 14 involves multiple parameters that have a non-linear relationship. Therefore, the control of the extent of coverage of the opening of the relay valve 14 is relatively complex, and a desirable level of control precision is not achieved, so there is room for improving the relay valve 14.
A conventional relay valve, as disclosed in U.S. Pat. No. 7,577,509, is designed such that flowing of pressurized air from an air reservoir to an antilock braking system is controlled by the amount of pressurized air flow through a solenoid controlled valve. However, an extent of coverage of an opening (not shown) of the conventional relay valve is also controlled by pneumatic pressure (i.e., the pressurized air flow), and thus such conventional relay valve has the same drawbacks and the same control precision issue as previously mentioned.